In the oil and gas exploration and extraction industry, a range of tubular strings are used to, for example, support tools and devices in wellbores, or convey fluid and other tools and devices between surface and downhole locations. Such tubular strings include: drill strings, used for supporting a drill bit and other drilling apparatus; casing and liner, used to line and seal a wellbore, and completions, used to carry oil and gas to surface. A string may be provided with a closable port in the wall of the string, to permit fluid communication through the wall. Typically, such a port will be closed by an axially movable sleeve. Seals will be provided between the sleeve and the string wall. At least one of the seals will be crossed by a port as the sleeve moves to open and close the port.